More specifically, the invention relates to an improved method of generating oxygen from air by air-oxidizing a suitable molten alkali hydroxide at least partially to superoxide and subsequently reversing the reaction, thereby obtaining nitrogen-free oxygen and regenerating the molten hydroxide.
Concentrated oxygen with none or little of the air's nitrogen is now widely in use and holds promise, if generated at low cost, for such applications as auto-thermal hydrogen generation from fossil fuels and for use as fuel cell cathode feeds and the like. Presently, relatively expensive cryogenic separation of the air's predominantly oxygen and nitrogen is widely practiced. Defining, for short, a “frigorie” as a negative calorie (i.e. the amount of heat necessary to be removed to cool one gram of water at 15° C. to 1° C.), all other things being equal, heat input is preferable to cooling, because frigories are significantly more expensive than calories. One principle of a known heating technique has involved a selective chemical oxygen acceptor at a moderately elevated temperature and releasing oxygen therefrom at a higher temperature,
By way of such examples, the prior art has illustrated the principle of extracting the oxygen from air as early as in 1897 (E. B. Stuart's U.S. Pat. No. 588,615) by air oxidizing “a manganate and a neutral salt capable of fusing and remaining in permanent liquid form” (such as a salt, e.g. a chloride or sulfate) and “subjecting steam” thereto. Similarly, L. G. Jenness's U.S. Pat. No. 2,486,530 (1942} relies for the same purpose on the reversible reactionNa2Mno4+H2O=2NaOH+MnO2+½O2.  (1)
S. A. Guerrieri's U.S. Pat. No. 3,310,381, as another example, describes oxidizing barium oxide to peroxide, as in the range of 500° C. to about 720° C., and removing high purity oxygen from the peroxide; for example, in the range of 700° to about 850° C. Here, factors such as corrosion, slow kinetics and incomplete separation have, however, been serious drawbacks.
Low cost potassium superoxide is also a well known oxygen donor which has the important advantage over the above “heat-reversible” oxides that water liberates its oxygen rapidly even at room temperature, reverting KO2 back to 2KOH.
The syntheses of potassium superoxide, KO2, and cesium superoxide, CsO2, by oxygen in their molten hydroxides are well known in the prior art, as shown in “Peroxides, Superoxides, and Ozonides of Alkali and Alkaline Earth Metals” by Il'ya Ivanovich, (Translated from Russian by J. Woroncow; Edited by A. W. Petrocelli), Plenum Press, New York, 1966, pages 105 and 112, respectively. The equilibria at varying temperatures of the endothermic reaction2KOH+1.5O2=2KO2+H2O,  (2)have been published by Von Hermann Lux, Rudolf Kuhn and Titus Niedermann (Zeitschrift fur anorganische Chemie, Vol. 298, 285-301), showing increasing peroxide concentrations up to about 600° C., low KOH amounts relative to oxygen flow rates and low steam concentrations. (The lowering of superoxide generation above 600° C. has been attributed to beginning decomposition.) Of special relevance to the present invention is the finding in this publication that the oxidation rates are in the order of many hours to reach equilibrium; this being further aggravated when air is used in lieu of oxygen, as hereinafter proposed for the purposes of the present invention.
With respect to improving the oxidation kinetics, reference is made to U.S. Pat. No. 3,471,332 (1969) to R. J. Allen, R. L. Novak and H. G. Petrow. It describes the catalytic enhancement by, for example, oxides of manganese, iron and cobalt, of the rates of cathodic oxidation of molten KOH—containing electrolyte of a fuel cell.
As later more fully detailed, this invention makes use of such molten alkali hydroxide(s) in a novel method of generating concentrated oxygen from air by at least partially oxidizing the molten alkali hydroxide to superoxide, then reversing the reaction to obtain nitrogen-free oxygen, and then regenerating the molten hydroxide.